1. Field of the Invention
The present invention relates to a method for recovering tantalum, and in particular to a technology for effectively recovering tantalum from tantalum-containing waste obtained from printed wiring boards containing tantalum capacitors or from a tantalum sintered compact collected from tantalum capacitors.
2. Description of the Related Art
Tantalum capacitors have high capacity and high stability, and many of them have been used especially for laptop computers, communication devices, large-sized computers and audio equipment and the like. And it is said that as many as about 3.8 billion tantalum capacitors are produced annually in Japan. These tantalum capacitors are distributed to the market with being mounted on various electronic and electric substrates such as printed wiring boards, but used tantalum capacitors are not actively recovered or recycled for economical or technical reasons and merely disposed as industrial waste at present.
Recently due to the problem of resource scarcity, efforts have been underway as a national project to recover and reuse rare metals from electronic and electric devices disposed. It is expected to recover with high efficiency tantalum which is one of the rare metals from waste substrates such as printed wiring boards and used tantalum capacitors.
As a conventional technology for recovering tantalum from tantalum capacitors, for example, a method of recovering tantalum from scrap tantalum by removing a manganese dioxide solid electrolyte, which is a coating material in used tantalum capacitors, by acid leaching, carbon reduction-acid leaching, or chlorination distillation or an argon-hydrogen plasma method and then refining it by a chlorination method, an alkoxide method, an electron beam method or a combination of these is known (see Patent Literature 1).
Also, a method for recovering tantalum comprising heat treating a waste substrate packaged with a tantalum capacitor to 550° C. or higher under an oxidizing atmosphere, and sorting the resulting heated product according to the major axis length is known (Patent Literature 2).
These prior art technologies enable a certain quality level of metal tantalum and tantalum oxide to be recovered from waste substrates and used tantalum capacitors. However, it is said that since tantalum recovered with these technologies contain a large amount of impurities such as silicon (Si), antimony (Sb), phosphorus (P), manganese (Mn), tin (Sn), lead (Pb), zinc (Zn), iron (Fe), nickel (Ni) and copper (Cu), direct reuse is difficult. For example, when designing recycling from waste substrates such as printed wiring boards, a large amount of impurities such as copper wire other than tantalum is inevitably included, and therefore it was difficult to address the problem by prior art technologies as described in Patent Literature 1.